PEG-modified G-CSF and PEG-modified human granulocyte colony stimulating factor derivative ND28 JP-A-316400/89 (the term "JP-A" used herein means a published unexamined Japanese Patent Application.) corresponding to European Patent No. 0335423, WO090/06952; hereinafter referred to as PEG-modified ND28! have advantageous properties such as a longer half life in blood when compared with human granulocyte colony stimulating factor (U.S. Pat. No. 4,883,127; hereinafter referred to as G-CSF) and human granulocyte colony stimulating factor derivative ND28 (JP-A-267292/88 corresponding to European Patent No. 0272703; hereinafter referred to as ND28), and thus have high clinical efficacy. In order to use PEG-modified G-CSF and PEG-modified ND28 clinically, a specific quantification system using an antibody is necessary in order to ascertain the pharmacokinetics in blood. However, the PEG-modified molecule is not readily recognized by the immune mechanism of a host, making it difficult to construct an antibody to it LYMPHOKINE AND CYTOKINE RESEARCH, 10, 475-480 (1990)!. Although monoclonal antibodies against G-CSF and ND28 have been obtained anti G-CSF monoclonal antibody: JP-A-180860/88, J. Immunol. Methods, 128(2), 211-217 (1990), J. Biol. Chem., 266(35), 23815-23823, anti ND28 monoclonal antibody: JP-A-225495/89 corresponding to European Patent No. 0331186, Agric. Biol. Chem., 53, 1095-1101 (1989)!, to date no antibodies which react with PEG-modified G-CSF or PEG-modified ND28 have been obtained.